Adult dosage calculations: the formulas that actually matter
Most dosage errors come from one of 3 things: wrong formula, wrong unit, or wrong concentration. This guide covers all 3.
Tablet and capsule dose calculations
The formula is: tablets to give = ordered dose / tablet strength. Say a doctor orders 750mg and you have 250mg tablets. That's 3 tablets per dose. Simple.
Where people go wrong is decimal placement. 0.75g ordered, tablets in mg. Always convert to the same unit before dividing. 0.75g = 750mg. Then calculate.
Liquid medication dose calculations
Volume to give (mL) = ordered dose (mg) / concentration (mg per mL).
But most suspensions are labelled as mg per 5mL, not mg per mL. So convert first: 125mg/5mL = 25mg/mL. Then if you need 250mg, that's 250 / 25 = 10mL. Confirm with a measuring syringe. Never use a kitchen spoon.
Weight-based dosing
Some drugs are dosed by body weight. The formula: ordered dose (mg) = dose in mg/kg x patient weight in kg. A 70kg patient prescribed amoxicillin at 25mg/kg/day gets 1750mg daily. Split across 3 doses, that's roughly 583mg per dose.
Two things to always check. First: is the dose mg/kg/day or mg/kg/dose? That changes everything. Second: does the calculated dose exceed the adult maximum? If it does, cap it at the maximum.
Injectable and parenteral dose calculations
Volume to draw up (mL) = ordered dose / concentration (same unit as ordered dose). If gentamicin is ordered at 80mg and the vial is 40mg/mL, you draw up 2mL. Double-check the vial label every time. Concentration labelling varies between manufacturers and between preparations.
When to pair this with other tools
Patients with kidney disease need dose adjustments before you apply any of these formulas. Run the Renal Dose Adjustment Calculator first, then come back here with the adjusted dose. For drugs where you need to verify you're within safe limits, use the Max Daily Dose Checker. And always check for interactions with the Drug Interaction Checker before administering anything new.